Creep-feed grinding is a full depth or full cut operation that often allows a complete profile depth to be cut from a solid material in a single pass. For smaller workpieces, the material to be machined is fed past a rotating grinding tool, typically a grinding wheel, at a constant speed. For larger pieces, the material to be machined can remain stationary and the grinding tool can be moved.
A high removal rate can be achieved using creep-feed grinding, but the process can generate sufficient frictional heat to burn the workpiece surface and damage the wheel. Coolant liquid is typically supplied to the grinding tool contact region ensuring workpiece cooling and grinding tool cooling and efficient cleaning. It is known to use nozzles having one or more jets to deliver coolant to the wheel surface in large volumes.
Removal of metal or other material from a workpiece at high rates can require a significant quantity of coolant that must be delivered precisely and in sufficient quantities at, and across the entire profile of, the interface between the metal working tool and the workpiece. Typically, the coolant nozzle is positioned manually by an operator based on experience and an estimate of an orientation and position that will deliver the coolant stream at the metalworking tool. The significant volume and pressure of the stream of coolant during a grinding operation, for example, floods the grinding compartment and obscures any view of the exact position of the coolant stream's impact and of the machining interface. Often, if the coolant stream has not been precisely delivered to the machining interface, the machined workpiece will have flaws due to excessive heat buildup or material removal, and must be reworked or scrapped.
It is sometimes desirable to use creep-feed grinding to form complex shapes such as re-entrant shapes, which are forms that are wider at the inside than it is at the entrance (e.g., a dovetail joint). Turbine components, such as jet engine, rotors, compressor blade assembly, typically employ re-entrant shaped slots in the turbine disks. The re-entrant shape is used to hold or retain turbine blades around the periphery of turbine disks. Mechanical slides, T-slots to clamp parts on a machine table also use such re-entrant shaped slots.
This type of form cannot generally be created by grinding with a large diameter wheel operated perpendicular to the surface of the part because it would be impossible for the wheel to enter the wider part of the form without removing the narrower part of the form. Instead, these types of features, such as for example the re-entrant shaped slots used to hold or retain turbine blades, can be formed in a two-step process. First a slot is formed into the workpiece, and then a finishing process can be conducted to change the contour of the slot to a complex shape (e.g., re-entrant shape). Instead of a perpendicular grinding wheel, the slot finishing process can be processed with a mounted-point grinding tool that extends into the slot and rotates in a direction substantially parallel to the surface of the workpiece.
In forming re-entrant shapes via creep-feed grinding, one common problem is that it is difficult to position coolant nozzles so that the coolant reaches the entire grinding tool/workpiece interface. Because the shapes are wider inside than at the surface, nozzles located above the surface of the workpiece cannot be directed at the entire interface between the grinding tool and the workpiece. As a result, nozzles are typically mounted so that they are aimed at either end of the slot to be machined, with a first nozzle at the front of the tool (so that on a first grinding pass, the tool is moved toward the first nozzle during grinding) and a second nozzle located behind the tool (so that the tool is moved away from the second nozzle). Significantly, the nozzles are mounted so that they retail a constant orientation with respect to the workpiece, but the distant between the nozzles and the grinding tool changes constantly during the grinding process.
Unfortunately, large coolant flow rates and pressures are required to make up for the distance traveled by the coolant when the tool is farthest away from a given nozzle. This results in both increased coolant usage and a requirement for more sophisticated coolant delivery systems.
Thus, the industry continues to demand improvements in the delivery of coolant to grinding tools.